1. Field of the Invention
The invention relates to galvanic cells of the zinc-anode type (e.g., zinc-air button cells) and in particular to miniature cells including those of gas and energy generation.
2. Background
Miniature button cells of various electrochemical systems such as Zn--O.sub.2, Zn--Ag.sub.2 O, Zn--HgO, or Zn--MnO.sub.2 are known for many years. Amongst them, Zn--O.sub.2 (air) cell has gained significant popularity because only the anode reaction material needs to be stored in the cell, whereas the cathode reaction material is oxygen, which is drawn from the surrounding air environment. Hence, the capacity and energy density of the zinc-air cell are determined by the stored amount of zinc metal and electrolyte solution in the cell.
The construction features of zinc-air button cells are quite similar to those of other commercially available zinc anode-based button cells. The zinc anode material is generally a loose, granulated powder mixed with gelled electrolyte to immobilize the composite and ensure adequate electrolyte contact with zinc particles. The two metal can halves, housing the cathode and anode active materials also act as the terminals, insulation between the two containers being provided by a plastic grommet. The top cap is of complex structure, pressed generally from a triclad metal sheet: outer surface is a protective layer of nickel over a core of steel. The inner surface that is in direct contact with the gelled zinc anode is high-purity copper or tin. The cathode sheet electrode is consolidated into the positive can, which is formed from nickel-plated steel having one or more holes to provide a path for oxygen to enter the cell and diffuse to the cathode catalyst sites. The cathode sheet structure includes catalyst layer, metallic mesh, hydrophobic membrane, diffusion membrane and air-distribution layer. The catalyst layer contains carbon, usually blended with oxides of manganese or silver. It is made hydrophobic by addition of finely dispersed polytetrafluoroethylene ("PTFE") particles. The metallic mesh provides structural support and acts as the current collector. The hydrophobic membrane maintains the gas-permeable waterproof boundary between the air and the cell's electrolyte. The diffusion membrane regulates gas diffusion rates. Finally, the air distribution layer distributes oxygen evenly over the cathode surface. It should be pointed out that one of the main construction features of commercially available button zinc-air cells is the presence of a separator to keep the anode and cathode separated. The use of separator is required as a consequence of the fact that the zinc anode is in powdered gelled form and therefore it must be separated from a direct electrical contact with the cathode.
Another fact related to the construction of button type zinc cells should also be pointed out. The zinc anode is in the form of gelled powder not only in zinc-air button cells (see e.g., U.S. Pat. Nos. 4,054,726, 4,189,526, 5,242,763, 5,279,905, 5,308,711, and 5,451,473), but also in other types of zinc anode-based button cells such as zinc-silver oxide (see, e.g., U.S. Pat. Nos. 4,021,598, 4,038,467, 4,068,049, 4,139,683, and 4,144,382) or zinc-carbon alkaline cells (U.S. Pat. Nos. 3,956,018, 4,091,186, 4,136,236, 4,192,914, 4,376,810, 4,617,142, 4,735,876, and 5,378,562). There are also several foreign patents in which zinc anode is also in powdered form (Japanese Patent publications 2236973, 6208450, 6273565, 61253764, and 62243252, and European Patent publications 768,723 and 414,990). In addition, the zinc powder is usually amalgamated to reduce gassing. Since mercury can be environmentally deleterious, significant efforts have been made to reduce the quantity of mercury in such cells. The most common way to reduce the gassing rate is by alloying the zinc powder with metals such as lead, cadmium, indium, bismuth, gallium, aluminum, tin etc., or adding oxides and/or hydroxides of these metals into the gelled powder mix.
In the absence of oxygen (or when a sintered PTFE membrane is utilized as the gas diffusion membrane), the zinc-air cell functions as a hydrogen gas generating cell as shown by Winsel in U.S. Pat. No. 5,242,565. In Winsel's button cell, the zinc anode is also in the form of gelled powder and the cell contains a separator. However, regardless of whether the cell is to be used as a current source or a gas generator, it is preferable for the cell to be of simple construction, to have fewer components, ease manufacturing and to be environmentally friendly.